It is well known to employ various endoprostheses for the treatment of diseases of various body vessels. One type of endoprostheses is commonly referred to as a stent. A stent is a generally longitudinal tubular device formed of biocompatible material which is useful to open and support various lumens in the body. For example, stents may be used in the vascular system, urogenital tract and bile duct, as well as in a variety of other applications in the body. Endovascular stents have become widely used for the treatment of stenosis, strictures or aneurysms in various blood vessels. These devices are implanted within the vessel to open and/or reinforce collapsing or partially occluded sections of the vessel. Often, stents may be used in conjunction with a graft with provides additional support for blood flow through weakened sections of the blood vessel.
Stents generally are open ended and are radially expandable between a generally unexpanded insertion diameter and an expanded implantation diameter which is greater than the unexpanded insertion diameter. Stents are often flexible in configuration, which allows them to be inserted through and conform to tortuous pathways in the blood vessels. The stent is generally inserted in a radially compressed state and expanded either through a self-expanding mechanism, or through the use of balloon catheters. For example, various stent constructions and their method of deployment are shown in U.S. Pat. No. 4,503,569 to Dotter; U.S. Pat. No. 4,733,665 to Palmaz; U.S. Pat. No. 4,856,561 to Hillstead; U.S. Pat. No. 4,580,568 to Gianturco; U.S. Pat. No. 4,732,152 to Wallsten and U.S. Pat. No. 4,886,062 to Wiktor. Published PCT Application No. PCT/US 95/08975, based on U.S. priority applications U.S. Ser. Nos. 08/282,181 and 08/457,354, also discloses a tubular shaped stent which is inflatable by balloon and which shrinks minimally in the longitudinal direction during expansion. The foregoing PCT publication and its U.S. priority applications, and the aforementioned U.S. patents are incorporated herein by reference. Additionally, published PCT Application WO 96/26689, entitled “Improved Longitudinally Flexible Expandable Stent” and being based on U.S. priority application Ser. No. 08/396,569 filed Mar. 1, 1995 and 08/511,076 filed Aug. 3, 1995 also discloses stents useful in the present invention, both this PCT Application and its U.S. priority applications being incorporated by reference herein.
The attachment of stents to grafts for use in endovascular applications has generally been by means of sutures, cuffs or pockets in the graft which serve to house the stent. For example, U.S. Pat. No. 5,522,881 discloses cuffs on the exterior surface of the graft which serve as open pockets into which stents can be placed. It is known to attach stents to grafts using sutures. For the most part, grafts which are used in combination with stents as composite device have been made from textile materials, which are woven, knitted or braided.
Composite devices made from stents and films have been disclosed in the art. For example, U.S. Pat. No. 5,123,916 to Lee describes an expandable intraluminal vascular graft which includes concentric cylindrical tubes having a plurality of scaffold members mounted therebetween. The scaffold members are expandable, ring-like and provide circumferential rigidity to the graft.
U.S. Pat. No. 5,383,926 to Lock, et al. describes a radially expandable endoprosthesis which comprises an elongated sleeve member in which the radially outward expansion of the sleeve is limited by connecting strips. These strips are selectively removable to allow further outward expansion. The sleeve can be C-shaped in cross-section to allow for further expanded growth. The sleeve member generally has an open wall structure such as those typical of wire mesh tubing or slotted tubing. An expandable sheet material may be disposed across the open region of the C-shaped sleeve member and may be formed of Gortex®.
U.S. Pat. No. 5,389,106 to Tower discloses an impermeable expandable intravascular stent. An impermeable deformable membrane interconnects portions of a distensible frame to form an impermeable exterior wall to the frame. The membrane is formed of a synthetic non-latex, non-vinyl polymer and the frame is made from a fine wire of annealed platinum. The distensible frame may be an expandable stent and the membrane is a hypoallergenic biologically inert material that is free of latex rubber proteins. The membrane should be impermeable and have the properties of elasticity, distensibility and barrier protection. No specific classes of materials are mentioned except the product name Tactylon®. The impermeable membrane is attached to the stent by dipping the stent into the polymer solution of the membrane and subsequently drying the device to remove the solvent. The stent is imbedded within the membrane surface.
With respect to grafts made from extruded materials such as expanded polytetrafluoroethylene (ePTFE), the use of sutures to attach such grafts encounters problems of dealing with bleeding through suture holes, since these expanded fluoropolymer materials do not generally have the self-sealing capability of elastomeric materials. Additionally, ePTFE is inherently resistant to adhesive bonding and few biocompatible adhesives will bond to its surface. While this inherent surface characteristic of ePTFE has advantages because it imparts a natural anti-thrombogenic characteristic to the surface of grafts made therefrom, it has been heretofore difficult to attach stents to grafts made from ePTFE without encountering the foregoing problems. The present invention seeks to overcome difficulties of attaching ePTFE material to a stent by using an anchoring material which can be carried into and entrapped in the porous surface of ePTFE.
In certain applications, it is necessary to protect against excessive cell growth through the stent (intimal hypoplasia), as well as thrombus formation and plaque buildup in the vascular system. In the bile or urogenital tract regions, tumor growth is also of concern. Additionally, arterial build-up of plaque and other debris can become dislodged from the vessel surface during or subsequent to implantation of the stent. To prevent such occurrences, the use of a cover or liner in combination with an ePTFE graft has been suggested by co-pending and co-assigned U.S. application Ser. No. 08/720,091 filed Sep. 27, 1996, and entitled “Improved Covered Stent”. This co-pending application is herein incorporated by reference. This copending application describes the use of unsintered ePTFE as a cover or liner used for a radially expandable stent. In the present application, a means of attaching such a cover or liner to a stent is provided.
An approach to preventing arterial buildup of plaque and other debris on an implanted stent has been discussed in co-assigned U.S. application Ser. No. 08/721,834, filed Sep. 27, 1996, entitled “Support Structure/Membrane Composite Medical Device”. This co-pending application is herein incorporated by reference. This application describes the use of support member/membrane composite device which includes a support structure such as a radially expandable stent, a porous polymeric non-textile membrane adjacent to said support structure which defines an interface therebetween; and a thermoplastic anchor means attached to and extending from said stent into said porous polymeric non-textile membrane at the interface in order to anchor the membrane to the support structure.
Another such approach is disclosed in U.S. Pat. No. 5,700,285 to Myers et al. in which two tubes are adhered through the openings of a fully encased stent. The two tubes are shown to be adhered together midway through each stent opening. The covered stent is formed at a first diameter and is radially collapsible to a second smaller diameter for delivery to a site within a body lumen. The ability of the covered stent to recover to the first diameter provides sufficient compressive force against the vessel wall to hold the device in place. However, such a device presents slack in the outer stent covering when the device is recovered to a diameter that is less than the first diameter, i.e. the diameter at which the stent is formed.
In the present invention, an outer stent covering is adhered or otherwise affixed to an inner stent covering, i.e. a liner, at a location substantially coextensive with the inner stent surface. Adhering the coverings in this manner will minimize the disruption of the fluid contacting surface of the inner stent covering and thereby minimize the turbulence imparted to the fluid flowing therethrough. The present invention also teaches adhering the outer stent covering to the inner stent covering so as to maintain an airgap therebetween adjacent the stent structure and thereby provide domains of relatively high porosity for promoting neointima ingrowth. Furthermore, the present invention discloses a method for covering an expandable stent at a sub-nominal size in order to provide an expandable stent with a taut outer stent covering. As the sub-nominal covered stent is expanded to the nominal diameter for engaging a vessel wall, the outer stent covering will continue to have little or no slack, enhancing the compliance matching between the vessel wall and the pores of the outer covering. The sub-nominal size at which a device of the present invention is formed can be chosen for various vessel sizes having nominal given diameters.
In endovascular applications where the use of graft/stent combinations are increasingly important, recent studies have shown that at small diameters, e.g., 4 mm, grafts have not been successful in vivo. For example, both textile and polymeric grafts, i.e. PTFE grafts, when used alone suffer from kinking and radial collapse subsequent to implantation. When stents are used alone, that is without a graft, patency of the vessel is well maintained, but as mentioned above, excessive cell growth through the stent, as well as thrombus formation and plaque buildup is problematical. The present invention seeks to combine the advantages of a graft with those of a stent in a composite structure. The intraluminal composite structures of the present invention seek to provide the relatively smooth fluid-contacting surfaces of a graft with the structural support advantages of a stent. One objective of the present invention is to provide an intraluminal device which is particularly useful in small diameter applications, such as 4 mm diameter vessels, and which remains patent subsequent to implantation. Thus, the intraluminal composite devices of the present invention are designed to provoke a tissue response in the body that is normally encountered with a bare stent. This tissue response is one which promotes ingrowth and healing. The intraluminal devices of the present invention behave more like a stent than a graft, but have the advantages associated with the graft and can be used particularly well at the small diameter applications without suffering from the disadvantages associated with the individual use of a stent for a graft.